ES2543683T3 - Wind turbine blade - Google Patents

Abstract

A wind turbine blade comprising a multilayer composite structure that includes a first reflective layer; a second layer separated from said first layer, said second layer comprising a deposited fabric on which are a plurality of resistive analog circuit (AC) elements; wherein said AC elements are tuned to interact with said first layer to provide EM energy absorption over a desired frequency range; and wherein said AC elements occupy less than 70% of the surface area of said fabric.

Description

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DESCRIPTION

Wind turbine blade

This invention relates to wind turbines and wind turbine blades and the reduction in reflectivity of such wind turbine blades to electromagnetic radiation. Examples are shown in International Publications WO 2009/140949 and WO 2011/024009.

The objectives of renewable energy are based largely on wind energy, both on land and offshore. However, a significant percentage of all planning requests for new wind farms are rejected on the grounds that they will interfere with civil airport radars. The use of radar absorption materials (RAM) for wind turbines has previously been proposed to reduce the radar effect of ATC (Air Traffic Control) and ADR (Air Defense Radar) by reducing the RCS (Radar Cross Section). Wind turbine blades are highly specified to meet rigorous performance criteria, however, and the introduction of RAM in the blades ideally causes little or no modification of the structural properties and / or weight of the blade. This presents a significant engineering challenge.

The present applicant has made a report entitled “Design and manufacture of radar absorbing wind turbine blades - final report” as part of a study funded by DTI. This report indicates the construction of glass-reinforced epoxy (GRE) and GRE / foam sandwich of certain wind turbine blades and it is suggested that a Jaumann-type absorbent including a resistive layer to a specific separation of an electric conductor could be used perfect (PEC). The outermost GRE liner can be modified using a loss impedance layer and a reflector included in the inner surface of the sandwich construction. It is stated that the impedance layers can be made of fiberglass fabrics impregnated with a small amount of chopped carbon fibers. A layer of carbon fiber cloth can act as the PEC, inserted into the back of the GRP lining.

It is an object of the present invention to provide improved wind turbine blades.

According to a first aspect of the present invention, then, a wind turbine blade is provided comprising a multilayer composite structure that includes

a first reflective layer;

a second layer separated from said first layer, said second layer comprising a fabric deposited on which are a plurality of resistive analog circuit (AC) elements;

wherein said AC elements are tuned to interact with said first layer to provide EM energy absorption over a desired frequency range;

and wherein said AC elements occupy less than 70% of the surface area of said fabric.

The analog circuit (AC) layers refer to geometric patterns that are composed of conductive or resistive material. They are often defined by their conductance and effective susceptance, which together can be used to model the electromagnetic response of the layer. In this specification, therefore, the term analog circuit element is used to refer to a conduction pattern, the precise geometry and the conductivity of the material from which the absorption of an AC structure (comprising the AC element is allowed) ) to be tuned to a designated frequency or frequency range. Such elements have been previously proposed for applications such as anechoic chambers and invisibility coatings and are typically designed to absorb radiation in a wide range of frequencies. In preferred embodiments of the present invention, however, since absorption in a known frequency band corresponding to the ATC radar is desired, the absorption panel can be advantageously tuned to provide a desired level of attenuation at a particular frequency.

In one embodiment, the reflective layer may simply comprise a conductive layer, such as a substantially continuous carbon sheet. In other embodiments it may be necessary only that the layer is reflective at or around the desired frequency, for example 3GHz. In this case a frequency selective surface (FSS) could be used. A periodic metallic pattern would be suitable in certain embodiments and could be formed by deposition of silver, nickel or copper on a fabric substrate for example.

The use of analog circuit elements provides a controlled impedance layer and provides the advantage that the patterns produced can be adapted to provide absorption with the same desired frequency response despite variations in the separation between the first and second layers. This is advantageous in maintaining an effective absorption in the desired frequency band across large areas of the blade which, for structural reasons, can have variable structures and profiles.

In addition, the AC elements offer a number of parameters that can be varied (for example, shape, external dimension, track width, gap separation) to provide frequency tuning. This

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It provides an improvement in design flexibility and can be exploited to maintain absorption over a specific frequency range despite variable layer separation, while at the same time ensuring that the mechanical properties of the AC layer are compatible. with the integration in the wind turbine blade.

In one embodiment, therefore, the wind turbine blade includes a plurality of surface regions, the separation between the first and second layers that is different in different regions and wherein said AC elements have different geometries in said different regions. Different geometries may result from variation in dimensions or separation of the elements, but alternatively or additionally they may result in different forms. In a particularly preferred embodiment the AC elements comprise squares, which can be square loops or solid square patches or a combination of the two. Other shapes such as circles, stripes and crosses can also be used.

Therefore, in at least one different first and second regions, there is a first and second different separations between the layers and a different first and second AC geometries can be provided respectively.

Thus, despite the complex variation in the structure and materials of a typical wind turbine blade, the desired absorption can be provided through most, if not all, of the blade surface using only the two-way approach. layers (AC resistive / controlled impedance layer and reflective layer) noted above and described in more detail below.

In addition to hosting relative variations in layer separation, an advantage is also provided in terms of the absolute layer separations that can be achieved. The embodiments of the proposed arrangements and methods now allow the two layers to be separated by less than a quarter of a wavelength or even a tenth or twelfth of a wavelength of the incident EM energy that has a frequency at which absorption is desired.

A shovel will typically extend in a direction longitudinally from the base where the connection to a hub is made, to the tip of the blade. A transverse direction extends from the leading edge to the trailing edge. A combination of factors will influence the number and pattern of the different regions that make up the surface of the blade of any given embodiment. The surface of the blade can be divided into different regions in both the longitudinal and transverse direction and a grid or block pattern can result. The embodiments may have five or more or ten or more different regions along the profile longitudinally and three or more different regions in a transverse direction. In larger wind turbine arrangements, therefore, each blade may include 20 or more or 40 or more different regions.

It is not necessary for each different region of the blade to have a unique AC pattern or geometry and two or more different regions may be required or it may acceptably use the same impedance layer design. This can help limit the total number of different patterns or geometries required. Therefore, while the boundaries between adjacent regions typically define a change in CA design, they do not need the introduction of a new design and the total number of different designs may be less than the total number of defined regions. Certain embodiments employ ten or more different AC designs. It may be possible to maintain the same basic design of AC elements (for example, square loop) across the entire blade and achieve the required variation by adjusting the track width for example. Alternatively, to achieve the required number and performance of different design, it may be necessary to vary multiple parameters such as separation and external dimension or even vary the shape of the element.

The regions may be contiguous, however, there may also be discontinuities between the regions, for example, if absorption is not desired or is not required for a certain region of the blade.

The number and pattern of the different regions can be dictated to some extent by the construction of the blade, for example, where there is a "radical" change from a monolithic to a sandwich construction. There may be a certain amount of design freedom, however, for example, when dealing with blade construction parameters that vary gradually. In some embodiments, a single region can be defined and a single AC design can be used despite some variation in the separation of layers across the region. This may cause absorption performance that is slightly suboptimal, but still acceptable. AC designs that offer good absorption over increased bandwidth offer an advantage in this regard. It will be understood that while dividing such a region into one or more smaller regions (i.e. finer regional resolution), each one having specifically adapted CA patterns could result in an improvement in absorption but at the cost of an increase in complexity .

Preferably the AC elements have an external dimension of less than 50 mm. The external dimension is typically the maximum external dimension, for example, the length of the side for a square. By maintaining relatively short lengths of conduction pattern, the layer is able to better tolerate bending without damaging the electrical properties of the elements themselves. If significantly longer driving elements are used, there is an increased risk of damaging the elements when the layer is formed in a curved blade profile.

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In preferred embodiments the analog circuit elements are formed of resistive material, such as a carbon-based composition. The AC layer is provided by a fabric (for example, woven glass fabric) in one embodiment, with the AC elements deposited on the fabric by screen printing with resistive ink. The resistive ink is preferably a carbon-based ink having graphite particles suspended in a binder or solvent. Such inks have often been considered difficult to work with and significant efforts have been made to facilitate inkjet printing of conductive and resistive inks or to find suitable alternative deposition techniques (eg, electrolytic metallization). It has been found, however, that screen printing is particularly suitable for the formation of an AC layer suitable for use in embodiments of the present invention. Screen printing is capable of depositing the ink in sufficient thickness (for example, 10-15 μm on polymer substrate) to achieve the required electrical properties. In addition, the thickness and volumetric resistivity can be controlled with precision and achieved reliability and consistency across a large area by selecting the appropriate parameters such as screen mesh type, scraper pressure, curing conditions, etc. It has been found desirable for AC elements to be

provided having a surface resistance of less than 80  / square more preferably 10 and 40  / square.

It has been found by the present invention that the percentage of fabric coverage by the AC elements should be controlled in order to ensure structural integrity within the composite shovel structure. Preferred embodiments therefore have the property that AC elements occupy less than 70% of the surface area of said fabric. Again due to the use of AC elements and the design flexibility associated with them, this can be achieved even when multiple other restrictions such as layer separation and absorption frequency are imposed.

Preferably the absorption provides an attenuation greater than or equal to 20 dB at the center frequency (of the desired frequency range). Since the absorber is tuned to this center frequency, the attenuation falls out of this frequency, however, to allow manufacturing tolerances and other practical considerations and an attenuation greater than or equal to 20 dB is typically provided in a bandwidth of approximately 300 MHz centered around the nominal. Typically beyond 100 MHz any side of the center frequency attenuation falls below 20 dB and can be 10 dB or less.

A further aspect of the invention provides a method of manufacturing a wind turbine blade comprising the screen printing of a fabric substrate with a plurality of resistive analog circuit (AC) elements; provide an electromagnetically reflective fiber fabric or layer; and including said printed fabric and said reflective layer in the whole of the composite layer structure of a wind turbine blade, so that said fabric and said resistive layer are separated by a defined separation.

The basic assembly technique of the composite layer structure can be by any conventional method such as pre-impregnated accumulation or resin infusion. The printed fabric and the reflective layer (which may be previously impregnated with resin) are inserted at the desired points in the sequence of layers so that, on the finished blade, they have the desired separation.

Preferably the screen printing is rotary screen printing and preferably the AC elements are formed of a resistive carbon based ink. In embodiments where different geometries of AC elements are used for different facets of the blade, separate sheets of fabric with different patterns can be printed and included in the assembly process in different corresponding locations. Fabrics that have different patterns or printed geometries can be placed directly adjacent to each other if desired. Alternatively, a single fabric substrate may be the object of gradual printing, with the pattern or geometry applied that varies across the fabric.

The invention extends to methods, apparatus and / or use substantially as herein described with reference to the accompanying drawings.

A feature in one aspect of the invention can be applied to other aspects of the invention, in any suitable combination. In particular, aspects of the method can be applied to aspects of the apparatus and vice versa.

In addition, the features implemented in hardware can generally be implemented in software and vice versa. Any reference to software and hardware features herein should be interpreted accordingly.

Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a cross section through a wind turbine blade.

Figures 2 and 3 show alternative shovel frame constructions.

Figures 4 and 5 illustrate examples of analog circuit patterns.

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Figure 6 shows multiple different facets of a blade portion.

The wind turbine blade 100, shown in cross section in Figure 1 is generally hollow under construction, formed by an outer frame 102 and which includes one or more elements or souls 104 to provide additional structural integrity. The outer lining is composed of multiple different sections, which have different constructions. For example, section 106 is formed of monolithic glass reinforced epoxy (GRE) while region 108 is formed of a GRE foam sandwich construction.

These two types of structure are illustrated in greater detail in Figures 2 and 3. Figure 2 shows a monolithic section 202 of GRE, which has a typical construction composed of multiple layers or sheets of fiberglass fastened in a matrix of polymer, as is well known. A controlled impedance layer 220 is included near the outer surface 204, while deeper in the structure, closer to the inner surface 206 is a reflective layer 230. These two layers are incorporated into the composite structure. Both layers 220 and 230 are flexible fabric based layers in preferred embodiments and thus can be included in the manufacturing process of composite material with minimal breakage or alteration of the existing manufacturing technique. The placement of the layers and hence their separation is determined by the sequence in which they are included during manufacturing.

Figure 3 illustrates a composite sandwich construction in which a foam core 312 is sandwiched between the upper and lower GRE liners 314, 316. Again, a controlled impedance layer 320 and a reflective layer 330 are included. , which are incorporated into the upper and lower (or inner and outer) GRE liners respectively, in an equivalent manner as described above. Here the separation between layers 320 and 330 is determined by the thickness of the foam core in addition to the position of the layers within the GRP portions.

Figure 4 shows a first example of an analog circuit pattern that can be used to provide a controlled impedance layer. Here each AC element is in the form of a square loop, the geometric parameters that are the external lateral dimension 402, the track width 404 and the gap 406 between each element. Only four elements are shown but it will be understood that the pattern extends across the entire region of interest. Figure 5 shows a second example of AC pattern, in which each element is a solid square patch. In this case the geometric parameters are simply the lateral patch dimension 502 and the gap between the elements 504.

Figure 6 is a perspective view of a section of an elongated wind turbine blade 600. The blade extends in a longitudinally direction 602, with the edge 604 of the illustrated section which is closer to the base of the blade and with the profile that generally narrows to edge 606 that is closer to the tip of the blade. A transverse direction is illustrated by arrow 608 from leading edge 610 to trailing edge 612.

As discussed above, the surface or liner of the blade can be divided into discrete regions as schematically indicated in 620, 622, 624, etc., for example, different regions having different AC geometries that make up a controlled impedance layer. . The edges of the regions may result from or correspond to a change in the underlying construction, for example a change from a solid lining region to a sandwich construction area. Alternatively the edges can define discrete ranges of values in a parameter that varies continuously, for example the thickness of an underlying epoxy layer that varies along or across the profile of the blade.

Therefore, in the example of Figure 6, moving in a profile along the transverse direction, the transition from region 620 to 624 is indicative of a change in cross section at the edge of a reinforcing core (for example, 104 in Figure 1). In the transition, the AC geometry of the impedance layer changes from a first pattern to a second pattern to maintain a desired absorption characteristic. Similarly, the transition from region 624 to 626 indicates the other edge of the soul. Again there is a change in AC geometry that can return back to the first pattern or a third pattern, different from the other two.

In the longitudinal direction, the transition from region 620 to region 622 does not indicate a radical change in construction, but results from a continuous variation in the properties of the underlying material in that direction. The position of the boundary between region 620 and 622 is determined as the result of a user-defined optimization process that aims to balance the absorption performance (for example, the attenuation value and bandwidth) versus complexity AC design.

It will be understood that the present invention has been described above purely by way of example and a modification of detail can be made within the scope of the invention.

Claims (1)

1. A wind turbine blade comprising a multilayer composite structure that includes a first reflective layer; a second layer separated from said first layer, said second layer comprising a fabric deposited on which are a plurality of resistive analog circuit (AC) elements; wherein said AC elements are tuned to interact with said first layer to provide EM energy absorption over a desired frequency range; and wherein said AC elements occupy less than 70% of the surface area of said fabric. 6